I agree that the 3:1 ranking is probably the major issue, along with two identical scores at the lower end of the range. Don't despair though!
Sit back and look at everything you have, especially those teacher statements. You have probably "read them to death" by now - perhaps enlist a trusted friend to re-read them with you. Take all the most positive statements from them and highlight those when talking to the panel.
Then look at the school books - again with a friend if it helps. A fresh pair of eyes can do wonders at this stage. Flag up with post-it notes the very best pieces of work. Decide if there are one or two that you can refer to in your appeal, so the panel really are forced to go looking for them at the end.
On the broken arm, you were "caught between a rock and a hard place". Go with the original date, when your daughter was moderately fit, although would have given a slower performance because of the temporary disability? Or wait six weeks, but with no certainty that the arm wouldn't have been an even greater hindrance or perhaps a bar to taking the exam at all? Your daughter's enthusiasm tipped the balance. BUT neither solution was ideal, and will have contributed to her lower marks.
On the ranking, the Head is new. His/her rankings produced some random results. Draw that to the panel's attention, but do not dwell on it! Also highlight that the OS was drawn up on Year 6 work, which in your daughter's case was a mere 3 weeks or so before she broke her arm. hardly a reliable "guide to future performance"!
Make sure that you highlight phrases from the medical reports, and make sure that the panel actually find that part of your evidence before you continue talking.
It's all about drawing their attention to the strong points in your case, and making them sink in aurally and also visually if possible.
Lastly, prepare (if you haven't already) or revisit the summary notes of your "speech" to the panel that you will give to the Clerk. They don't just earn you brownie points, they are a useful record of what you have said, and the panel can ask the Clerk to refer back to them if needed during their deliberations.
Finally, on your own or with the trusted friend, do a Waffle & Whinge Check! (A new invention of mine, to add to the post-it notes!)
Decide frankly where you are over-doing the mitigating circumstances ("the cold had such a dreadful effect on her attitude to school over several weeks"), where you are overdoing any extraneous points ("such a helpful girl, always holds doors open for younger children") - you know the sort of thing by now. Even the strongest case has a few of those, but they can really ruin a good case. Less is usually more.